Comparison of covalently-bound aliphatic biomarkers released via hydropyrolysis with their solvent-extractable counterparts for a suite of Kimmeridgeclays

Fixed-bed hydropyrolysis at pressures greater than 10 MPa has the unique ab ility to maximise the yields of covalently-bound alkanes without adversely affecting their stereochemistries. In order to compare the yields and compo sitions of the solvent-extractable and kerogen-bound hopanes and steranes f or source rocks of varying maturity, six Kimmeridge clay samples (Jurassic, Type II kerogen) have been subjected to successive dichloromethane extract ion and hydropyrolysis. The ratios of free to kerogen-bound aliphatic bioma rkers increase markedly at relatively low vitrinite reflectance (ca. 0.45 0 .50%) with large reductions occurring in the concentrations of bound hopane s and steranes. However, the fact that the total concentrations of observab le hopanes and steranes (i.e. free plus kerogen-bound) do not decrease unti l the vitrinite reflectance is greater than ca. 0.50% provides evidence tha t neither oil migration nor aromatisation has begun to occur to a significa nt extent below this maturity level. To the authors' knowledge, this repres ents the first study to observe a systematic change in epimer ratios with i ncreasing maturity for hopanes and steranes covalently bound in kerogens. I ndeed, the kerogen-bound hopanes and steranes undergo the same epimerisatio n reaction pathways as their free counterparts in the bitumen, but they are generally less mature in terms of isomerisation at both ring and sidechain chiral centres. Indeed, the range of the moretane/hopane index for the hyd ropyrolysis oils is much larger than that for the bitumens (ca. 5 times), i ndicating a much greater sensitivity to relatively small changes in maturit y. The covalently-bound hopanes and steranes released via hydropyrolysis ha ve enabled the maturity of a contaminated core from the North Sea (Central Graben) maturity to be assessed and have showed that it is compatible with 'the Kimmeridge formation. (C) 1998 Elsevier Science Ltd. All rights reserv ed.